There are many situations where there is a requirement to deliver therapeutic agents to specific targets within the brain parenchyma via implanted catheters. Furthermore, many of these therapeutic agents will cause unwanted side effects if delivered to healthy parts of the brain. Examples of treating abnormalities of brain function include the acute infusion of Gamma-amino-buturic-acid agonists into an epileptic focus or pathway to block transmission, and the chronic delivery of opiates or other analgesics to the peri-aqueductal grey matter or to thalamic targets for the treatment of intractable pain. Also, cytotoxic agents can be delivered directly into a brain tumour. Intraparenchymal infusion can also be used to deliver therapeutic agents to brain targets that cannot be delivered systemically because they will not cross the blood-brain barrier. For example, the treatment of patients with Parkinson's disease, Alzheimer's disease, head injury, stroke and multiple sclerosis may be carried out by the infusion of neurotrophic factors to protect and repair failing or damaged nerve cells. Neurotrophins may also be infused to support neural grafts transplanted into damaged or malfunctioning areas of the brain in order to restore function.
It is also known to insert instruments other than catheters, such as electrodes, directly in the brain parenchyma. For example, stimulating and lesioning electrodes are used in a variety of surgical procedures, including deep brain stimulation (DBS) electrodes. A surgeon wishing to stimulate or lesion a particular area of nervous tissue can target the end of an electrode to the target site so that a desired electrical current can be delivered.
The above described methods rely on targeting the required site as accurately as possible. Slight misplacement of the instrument being inserted may lead to significant morbidity or treatment failure. For example, brain targets for treating functional disorders are usually deeply situated and have small volumes. A desired target for treating Parkinson's disease is situated in the sub-thalamic nucleus and is 3-4 mm in diameter, or an ovoid of 3-4 mm in diameter and 5-6 mm in length. Other targets such as the globus palladus or targets in the thalamus are usually no more than 1-2 mm larger. For such a small target sub-optimal placement of as little as 1 mm will not only reduce the effectiveness of the treatment, but may also induce unwanted side effects such as weakness, altered sensation, worsened speech and double vision. It is also desirable to minimise trauma in certain regions of the brain; for example, the mesencephalon (which includes the subthalamic nucleus, the substantia nigra and the pedunculor-pontine nucleus) is a critical region of the brain where is it is important to minimise trauma from the passage of an electrode or catheter.
A variety of stereotactic devices and methods have thus been developed previously in an attempt to allow instruments to be accurately guided towards a target identified by a surgeon (e.g. using x-rays or magnetic resonance imaging) with the minimum of trauma to other regions of the brain. Examples of prior systems are given in EP1509153, U.S. Pat. Nos. 6,609,020 and 6,328,748.
GB2357700 discloses a guide device comprising a port (head) which is secured in a hole formed in the skull with a tube of the guide device extending into the brain. In order to insert the guide device into the skull a hole is first drilled into the skull under the guidance of a stereotactic frame. Often, the port has a larger diameter than the catheter/electrode/guide tube to be inserted into the brain. Therefore, it is desirable to drill an opening having a stepped form with an enlarged diameter portion for receiving the port. Devices for drilling such a stepped form are known, for example as described in WO2011/110874.
To insert the guide device to the desired target the dura is perforated. The surgeon may do this with a scalpel or a device such as described in WO2009/047494.
Use of a scalpel or a spike, such as disclosed in WO2011/110874, to puncture the dura and the later insertion of probes, such as guide rods, guide tubes, catheters, in to the brain, especially through the denser parts of the brain, may result in the brain being displaced from its original position, resulting in the probes missing the target.
Furthermore, during formation of the openings in the skull and in the dura using these separate instruments, the openings may not be correctly located relative to each other. This is even the case if a stereotactic frame is used for positioning the instruments during formation of the openings as play in the stereotactic frame can result in errors in positioning. This may hinder insertion of an implantable instrument, such as a catheter with integrated port.